Open House New York

Learn the secrets behind NYC's buildings, parks and other sites; we've got the scoop on five tours, with things to do after.

Gowanus Canal Sponge Park

Meet at the corner of DeGraw and Nevins Sts, Gowanus, Brooklyn; Sat 10:30am; e-mail dlandstudio@gmail.com for reservationsThe gist: A park near the Gowanus Canal? Believe it. This tour offers a preview of long-term plans to build green space around the Superfund site. The 11-acre park, yet to break ground, would catch the sewage runoff that drains into the canal during rainstorms, and use it for irrigation. The tour will traverse the northern edge of the canal and point out the sewer overflows system and historic landmarks, such as the Carroll Street Bridge.Fun fact: Landscaping the industrial banks of the Gowanus will provide more than a face-lift. Certain plants, like sunflowers (which absorb metal ions) and duckweed (which processes PCBs), break down waste.Afterward: Leave the banks of the canal behind, and head to Four & Twenty Blackbirds (439 Third Ave at 8th St, Gowanus, Brooklyn; 718-499-2917), a small shop serving savory ($5--$6) and sweet ($4--$5) pie (flavors change seasonally).

Highbridge Park

Meet at the northwest corner of River Ave and 167th St, Bronx; Sat 1--4pm (tours every hour) The gist: With plans afoot to reopen the High Bridge and renovate the Manhattan side of the park, the Emerging New York Architects staged an international ideas competition for the lesser-known Bronx side. Architects from the organization will lead tours that discuss the winning designs and the potential of the hilly, lush land. For example, rainfall could be pumped to create waterfalls on the bridge, while space between its Roman-style arches might hold an art gallery.Fun fact: The High Bridge opened in 1848, making it the oldest bridge in the city.Afterward: Check out Lobby-for-the-Time-Being 2009--2010 at the Bronx Museum of the Arts(1040 Grand Concourse at 165th St, Bronx; 718-681-6800, bronxmuseum.org; suggested donation $3--$5), a shadow-casting installation by borough architect Vito Acconci.

Motor Vessel Red Hook: A Sludge Boat

Owls Head Wastewater Treatment Plant, 6700 Shore Rd at Bay Ridge Ave, Bay Ridge, Brooklyn. Sun 10:30am--1:30pm (tours every half hour); visit nyc.gov/dep for reservationsThe gist: This boat carries your shit—literally. The Department of Environmental Protection maintains three vessels that haul 2.24 million gallons of sewage sludge daily from wastewater-treatment plants to dewatering facilities in the city, which turn the waste into fertilizer. See the new 350-foot addition to the fleet, and its odor-control system, engine room and radar navigation equipment. Be sure to wear flat, closed-toe shoes. (Just in case.)Fun fact: The boat's cargo is less nasty than you'd think: It's already been heated and mixed for several days in oxygen-free "digester" tanks that convert organic material into water, carbon dioxide and methane gas. So you won't smell like poo when you leave. Hooray!Afterward: Meander through Owl's Head Park (enter at Shore Rd and 68th St, Bay Ridge, Brooklyn; nyc.gov/parks), and observe Tony Hawks--in-training at the Millennium Skate Park.

Islamic Cultural Center

1711 Third Ave at 97th St; Sat, Sun 11am--4pmThe gist: With all the brouhaha over the proposed Muslim cultural center downtown, this tour of Manhattan's only freestanding mosque feels especially apropos. Designed by renowned architects Skidmore, Owings and Merrill and completed in 1991, its prayer room is spare, with huge windows. During the 11am tour on Saturday 9, senior designer Mustafa K. Abadan will speak about the building.Fun fact: The mosque sits at a 29-degree angle to the street grid, which points it in the direction of Mecca.Afterward: Grab a belly-filling, cheesy calzone ($8--$17) at Queens import Nick's Pizzeria (1814 Second Ave at 94th St; 212-987-5700, nicksnyc.com).

Salmagundi Club

47 Fifth Ave between 11th and 12th Sts; Sat, Sun 1--4pm; call 212-255-7740, ext 200 for reservationsThe gist: Dip into rarefied classic New York in this beautiful 1854 brownstone, home to one of the country's longest-running arts clubs. Tour areas usually off-limits to the public—like the wood-paneled library (which was used as a stand-in for Yale University in the 2006 film The Good Shepherd)—and discuss the 137-year-old club's history and members (including painter William Merritt Chase).Fun fact: The club's moniker was inspired by Washington Irving's satirical, 19th-century magazine, Salmagundi Papers; it's also the name of a stew that's still served in the club's dining room.Afterward: Fuel up with a potent cappuccino ($3.50--$4) from Joe the Art of Coffee (9 E 13th St between Fifth Ave and University Pl, 212-924-7400).

EXPLORE THE CITY! Open House New York is Sat 9 and Sun 10; call 212-991-6469 or visit ohny.org for details.

WIN TIX! Win a Weekend Passport to OHNY, which lets you jump the line at events that don't require a reservation. Click here to enter.